Alarm system with central station monitoring of key operated locks of a number of remote stations

ABSTRACT

An alarm system with a plurality of sensors at a protected premises, a transmitter circuit connected to the sensors for transmitting a pure sinusoidal signal whose frequency identifies the premises and for ending transmission or modulating the sinusoidal signal whenever one of the sensors detects an alarm condition, a central station connected to a number of receivers for providing an alarm signal when a sensor detects an alarm, a telephone line connecting the transmitter to the central station and an activating circuit at the protected premise including a reed switch mounted within a housing on the interior of the door with a momentary switch and light mounted on the same housing, a key operated tumbler having a magnet mounted thereon on the exterior of the door opposite the housing for closing the reed switch to activate the system.

United States atfet 91 @estreieher Jan. 29, 1974 Lesher 340/409 Smith 340/274 Primary Examiner-Donald J. Yusko Attorney, Agent, or Firm-John W. Malley et a1.

[57] ABSTRACT An alarm system with a plurality of sensors at a protected premises, a transmitter circuit connected to the sensors for transmitting a pure sinusoidal signal whose frequency identifies the premises and for ending transmission or modulating the sinusoidal signal whenever one of the sensors detects an alarm condition, a central station connected to a number of receivers for providing an alarm signal when a sensor detects an alarm, a telephone line connecting the transmitter to the central station and an activating circuit at the protected premise including a reed switch mounted within a housing on the interior of the door with a momentary switch and light mounted on the same housing, a key operated tumbler having :a magnet mounted thereon on the exterior of the door opposite the housing for closing the reed switch to activate the system.

13 Claims, 8 Drawing Figures PATENTEBJANZQ I974 SHEET 3 OF 5 NMNNwNK UQ QkaAN s PAIENIED 3.789.382

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FED/g9 OUTS/17E PAIENIEB M29 i974 SHEU 5 [IF 5 ALARM SYSTEM WITH CENTRAL STATION MONITORING OF KEY OPERATED LOCKS OF A NUMBER OF REMOTE STATIONS BRIEF DESCRIPTION OF THE PRIOR ART SUMMARY OF THE INVENTION The invention relates to an alarm system for providing an indication at a central station of a detected emergency, such as intrusion, fire etc., at a protected, remote premise together with an indication of the premise location and emergency.

One type of alarm system which is commonly used today and which has found wide acceptance has a central station which is continuously manned and which is connected for example by telephones or the like to remote sensors at each of a plurality of protected premises so that when any of the sensors detects an alarm condition such as intrusion, fire, etc., a signal is generated which is transmitted to the central station and displayed so that the operator thereof can take appropriate action, such as calling the police, fire department, etc.

One common type of this system now in use employs a direct current signal which is coupled through a pair of conductors, frequently leased from a utility, to a meter and relay at a central station. In the normal condition, a specific current flows through the circuit and, upon activation of the alarm system, this current is interrupted, intensified or modified to cause a central stationsensing device, such as a relay, to be activated or deactivated, indicating an alarm. In this type of system, special telephones circuits are normally required in order to assure low and constant d.c. resistance throughout the circuit and further, only one premise can be served by each pair of lines and must bear the total cost thereof.

Another type of system now in use is designed to share the cost of the connecting lines among a number of premises, and connects each premise in a series or loop circuit of the line. Operation of sensors in any of the protected premises interrupts the series current path. A premise having an activated sensor is usually identified by the means of telegraphic or similar code which is applied to the line by a mechanical device. The drawback with this type of the system is the need for a relatively high voltage to pass sufficient current through the loop circuit resistance. Further, the loop can be overcome by short-circuiting at any individual premise.

Yet another approach which is described in further detail in the patent to Rusnak US. Pat. No. 3,357,009 is to use a number of separate leased telephone lines connecting each of a number of protected premises to a central station with each protected facility impressing a signal having a different frequency on the line so that the identity of the station can be ascertained by determining the frequency of the signal received at a central station.

Other patents in the prior art show other aspects of an alarm monitoring system which may be desirable. For example, the patent to Kinsey, US. Pat. No. 3,452,345 described an alarm monitoring system which provides that the alarm maintains itself after being operated until it is manually reset at a central fa cility. The patent to Glidden US. Pat. No. 3,390,234 describes a system in which a battery is included to 0perate the alarm in the event of power failure. The pa tent to Streit US. Pat. No. 3,523,162 describes a remote signal system which employs telephone lines to interconnect the central facility to a number of remote stations, and likewise employs a capacitor to delay operation of the alarm at the central facility to avoid false alarms due to transients on the lines, etc.

The present invention relates to an alarm system of the type generally described above which includes a central station connected to a number of remote stations at individual premises to be protected by leased telephone lines or the like. Each of the individual premises includes one or more transmitters which each apply to the telephone line continuously a pure sinusoidal of a different frequency. At the central control station, a plurality of receiver circwits each adapted to detect one of the pure sinusoidal frequency are attached to the line for providing an output signal should that frequency no longer be present on the line, indicating the occurence of an alarm condition. After the activation of the alarm device at the central station by the absence of any given frequency, the alarm remains activated until manually reset, regardless of whether the frequency should reappear. An audio, visual or other alarm can be given at the central station.

To differentiate between the different types of emergency signals, for example, fire and intrusion, an ancillary circuit can be provided with a second type of sensor at the transmitter for modulating the transmitter output following activation of a sensor so that an onoff signal is transmitted to the line. An audio or light can be provided at the central station for providing a pulsating signal indicating a different type of emergency condition.

The transmitter or transmitters at each individual premise are preferably activated by a unique arrangement which is preferably associated with the main door in and out of the premises. This arrangement includes a key operated tumbler and visible light on the outside of the door. A momentary switch and a second visible light are likewise mounted on a housing on the inside of the door, opposite from the tumbler and outside light. A reed switch or the like is mounted inside the housing and shifts from a first to a second position when a magnet mounted on the tumbler is rotated in response to manual operation of the tumbler with an appropriate key. The momentary switch inside the premises electrically connects the lamps inside and outside to the various sensors.

Thus, when the premise is to be activated, usually by the last person to leave in the evening, that individual simply operates the push button on the inside of the door before opening the same to leave. This operates to light the two lamps to indicate as'a test if the system is functioning properly and all of the sensors are operable. The door is then opened and the system activated after the door is reclosed by operating the tumbler mechanism on the outside with the key to shift the position of the reed switch and transfer the current path through said sensors to the transmitters of that premises which are applying their unique sinusoidal signals to the telephone lines. Interruption of this signal thereafter operates to cause a visual or audio alarm to be given at the central facility so that the monitoring individual can take whatever action is appropriate.

Many other objects and purposes of the invention will become clear from the following detailed description of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic of the novel system of this invention with a plurality of protected premises connected to a central station by a telephone line or the like.

FIG. 2 shows the transmitter and sensors in each premises for each applying a unique sinusoidal signal to the telephone line.

FIG. 3 shows a schematic of the circuit in the central station for detecting absence of a sinusoidal line and giving a visual or audio alarm.

FIG. 4 shows an alternative circuit for providing an alarm when only one type of emergency condition is to be detected in a given system.

FIG. 5 shows a perspective view of the activating circuit of this invention on the interior of the door.

FIG. 6 shows a perspective view of the activating system of this invention on the exterior of the door.

FIG. 7 shows a cutaway view of the activating system of this invention installed on a door as shown in FIGS. 5 and 6.

FIG. 8 shows an electrical schematic of the activating circuits illustrated in FIGS. 5 through 7.

DETAILED DESCRIPTION OF THE DRAWINGS Reference is now made to FIG. 1 which shows schematically the novel system of this invention for providing an alarm in the event of intrusion, fire or other emergency at any one of a number-of different premises. As shown in FIG. 1, a plurality of premises indicated as 20, 22 and 2.4 are connected by a pair of telephone lines indicated as 26 to a central alarm station 28 at which an individual is stationed to monitor all of the premises and to take whatever action is appropriate in the event of an emergency. Each of the protected premises includes, as shown in block 20, one or more transmitting circuits such as circuits 30, 32, 34,

36, 38 and 40 which are connected to the telephone line 26 to each impress upon line 26 a different pure sinusoidal signal when the system is activated. Each transmitter is connected to a plurality of sensors, such as sensors 42 connected to a transmitter 36, so that the transmitter ceases applying the sinusoidal signal in the event that one of the sensors detects an emergency condition, such as intrusion or fire.

In central station 28 a plurality of receivers such as receivers 44, 46 and 48 are connected to the telephone line with one receiver being provided for each transmitter in each of the premises of the system. Each receiver is responsive to a different frequency for providing an output signal when that frequency is no longer detected by the system. The absence of this signal indieating an alarm condition is displayed on a conventional display 48 which may be comprised of a plurality of lamps, with each lamp denoting the location of the associated premise and an alarm 50 activated to provide an indication that something is amiss.

One of the advantages of the arrangement illustrated in FIG. I is that the use of a pure sinusoidal signal affords high reliability in identifying a signal providing an emergency alarm signal. There is no need to sychronize transmission from different premises and the sinusoidal signals can be simply superimposed and easily separated at the central station. The system is simple and reliable in operation and can be installed by individuals with minimum skill. Since the tone is generated normally at all times, the failure of the transmitter or other circuitry provides an indication, thus providing failsafe capability. Likewise, an attempt to destroy or circumvent the alarm arrangement will itself provide an alarm at the central station.

Alternatively, instead of connecting each of the premises serially by a single line to the central station,

a separate line can be used to connect each of the stations directly to the central facility or, for example, to a telephone station with a trunk pair or the like then connecting that station either to another telephone exchange station or directly to the central station. Thus, the system can be used effectively for many different types of premises such as large office buildings, apartment houses, individual residences, small buildings and the like.

Reference is now made to FIG. 2 which shows a schematic of one embodiment of a transmitter circuitry which can be used with special effectiveness in the system illustrated schematically in FIG. I. A conventional A.C. power source 60 is connected to a main power supply and also supplies energy to an auxiliary battery 62 which provides reserve energy in the event that the power line is severed or power is otherwise lost. Transmitter circuit 64 includes a conventional oscillator comprised of transistors 66 and 68 as well as resistors 70, 72, 74 and 76 and a conventional frequency selecting circuit 80 which is used to adjust the output frequency of transmitter 64. The output of the oscillator circuit is applied to telephone lines 26 via a buffering amplifier 80 and imerface network transformer 82. Resistors 84 and 86 level the amplitude of the signal injected into the telephone lines 26 at the receiving end of the line to equal the level of other signals of different frequency injected into the circuit at the same or other points.

The output of amplifying transistor 66 is fed back to its input in degenerative fashion through selective frequency network 80, nulling at one frequency and in regenerative fashion through adjustable resistor 76. When the regenerative feedback exceeds the degenerative feedback, a pure single frequency wil be produced which is relatively immune to variations from external factors.

Other means for selecting and controlling the frequency of the sinusoidal signal may also be used, such as tuning forks, inductance-capacitance circuits, crystals (piezo-electric) or others.

Power supplies 60 and 62 are connected to transmitter circuit 64 for causing that circuit to apply its pure sinusoidal tone to lines 26 via activator circuit 90 which is described in greater detail below, switch 92, which is controlled by relay coil 94, the plurality of sensors indicated as 96, switch 98, controlled by relay 100, and capacitor 102. Activator circuit 90 operates as described below to provide a simple way in which the system can be activated at each individual premises, for example, by the last person leaving the premises at the end of the day. When activator circuit 90 is activated, power supply 60 is connected to switch 92 which is normally closed as shown. Switches 96 represent normally closed switches and are associated with sensors such as conventional intrusion sensor or the like so that in the event of an alarm condition the associated switch is opened, interrupting the current path from power supply 62 to transmitter 64 and thus ending the impression of the unique frequency signal produced by transmitter 64 onto lines 26. As described above, this causes an alarm in central station 26.

Switch 98 which is a normally closed switch is controlled by relay 100 which is in turn connected to the power supply 60 via the parallel connected switches indicated as 104. These switches are normally open sensor switches which close upon the detection of a given alarm condition, activating relay 100, which in turn opens normally closed switch 98 likewise interrupting the current path between power supply 60 and transmitter 64 which in turn ends the impression of the frequency signal produced by transmitter 64 onto lines As mentioned above, according to one aspect of the invention of this application, a different kind of signal can be impressed on line 26 to indicate a different type of alarm condition. For example, switches 96 might be associated with intrusion and other switches such as those indicated as 110 associated with fire. The opening of one of the normally closed switches 1111 acti vates a conventional timing circuit 112 which is connected to the power supply 60 via activator circuit 90. Activation of timer circuit 1 12 operates relay 94 which opens switch 92 periodically to cause transmitter 64 to operate on and off to produce a modulated signal on telephone line 26 which can be detected by the central station to produce a different type of indication so that the monitor at central station 23 will be aware that a fire or othertype of emergency exists. Many suitable types of timer circuits are commercially available.

Reference is now made to FIG. 3 which illustrates a schematic of the circuitry in central station 28 for receiving the sinusoidal signals upon telephone line 26 and determining when one of the different sinusoidal signals is no longer being impressed on line 26. The sinusoidal signals on line 26 are coupled to a line terminating network comprising transformer 121) and potentiometer 122 which can be manually operated to adjust the amplitude of the received signal. The output of potentiometer 122 is connected to lines 124 and 126 with momentary switch 128 being provided in series connection with line 124 as shown.

A number of different detector circuits including detector circuits 44, 46 and 48 as illustrated in FIG. 1 are connected to line 124 and 126 for receiving all of the different sinusoidal signals on those lines. Only detector circuit 44 is illustrated in detail and it wil be understood that circuits 46 and 48 are preferably similar and that one detector circuit would normally be required for each different frequency being produced by the premises protected by the system shown in FIG. 1.

The signals on lines 124 and 126 are first passed through a conventional isolating buffer circuit including transistor 134 and then to a filter circuit 136 which may be a bridged-T filter or tuning fork or the like and which provides passage only for a given frequency signal corresponding to one of the frequencies produced by one of the transmitters in the protected premises. The pure signal frequency output of filter 136 is amplified by the circuit including transistor 136 and converted by diode 140 and capacitor 142 to a direct current voltage, maintaining transistor 144 in its conductive condition.

Transistor 144, when conductive, grounds the gate input to conventional SCR 146 so that SCR 146 is normally non-conductive. However, in the event that the frequency to which filter 136 is tuned is no longer received, transistor 1 becomes non-conductive, applying a positive voltage to the gate of SCR 146 so that it is rendered conductive completing a current path through bulb or lamp 1511 via normally closed momentary switch 152 and resistor 153. Bulk 1511 is preferably mounted on a display panel with a number of other bulbs, with each bulb being labelled to indicate the premise with which it is associated. An illuminated bulb thus indicates to the monitor the location of a premise which is signaling an emergency condition.

Even if the signal to which filter 136 is responsive is thereafter reapplied to line 26, SCR 146 will remain conductive until momentary switch 152 is manually operated to interrupt the current path through it so that bulb 1511 remains lit once an emergency condition has been detected, irrespective of whether that indication is thereafter ended until switch 152 is operated.

Detectors 46 and 48 operate similarly, lighting lamps 164) and 162, which are connected in series with momentary switches 164 and 166, respectively. All of the switches and lights are preferably mounted on an appropriate conventional display.

In addition to lighting a lamp or operating another device providing an indication of which premise has signalled an emergency condition, the central station also preferably includes a circuit which provides an audio or visual alarm in the event that any of the stations in question exhibits an emergency condition. Upon the interruption or cessation of the selected pure frequency signal to which filter 136 is responsive and the corresponding rise in potential at the collector of transistor 146, that rise in potential is passed through capacitor 172 and differentiated by the action of capacitor 172 and resistor 1180, producing a transient surge potential which is passed through diode 182 and applied as an input to circuit 170. This surge of potential renders transistors conductive thus completing a current path through either lamp 192 or audible horn 194 via switch 196, depending on the position. of switch 196.

As indicated above, the circuit illustrated in FIG. 2 includes a circuit for providing a modulated on and off signal in the event of one type of alarm condition. When a modulated signal is received, transistor 190 will be alternately turned on or off so that a pulsating output is produced by light 192 or horn 194 indicating to the operator that a first type of condition exists. If a continuous output is produced, then the monitoring individual knows that the other kind of condition exists. In those types of installation where there is no need for morethan one type of signal, a circuit such as shown in FIG. 4 can be used in place of circuit 170. SCR 195 is activated by the surge potential applied by diode 192 in FIG. 3 and remains activated until normally closed momentary switch 197 is operated to interrupt the current path through SCR 195. As in circuit 170, a manually operable switch 198 connects either light 199 or horn 2111 in series with SClR 2119 to provide an alarm upon a detection by any of the detector circuits of an alarm condition.

Reference is now made to FIGS. 5-8 which show in detail one activator circuit which has been found to be particularly effective. As shown in FIGS. 5 and 6, this activator includes a housing 200 which is mounted on the interior surface of the door through which ingress and egress is usually made at a protected premise. A jewelled light 202 and a momentary push button 204 are mounted on the outside of housing 200 so as to be visible and operable from inside of the door.

As can be seen in FIG. 6, a plate 206 is mounted on the exterior of the door opposite the location of housing 200 and has mounted on it a similar jewelled lamp 208 and a conventional key operated tumbler 210. As can be seen in FIG. 7, conventional tumbler 210 connects to a conventional small bar magnet 2112 so as to rotate that magnet through an angle of 90 when tumbler 210 is operated with key 214. A conventional reed switch 216, as described in detail below with referece to FIG. 8, is mounted within housing 200 adjacent bar magnet 212 so that the rotation of magnet 2112 causes the reed switch to shift from a first to a second electri cal position, activating the system. A simple circuit is provided for effecting the functions of the activator and is mounted within housing 200 and connected to pushbutton 204.

Referring now to FIG. 8, reed switch 212 is shown mounted adjacent to bar magnet 212 which is operated by a key 214, manipulating tumbler 210. The activator circuit of FIG. 8 is provided with 4 terminals-A,B,C, and Dand the connection of these terminals in the embodiment of FIG. 2 is shown. From FIG. 2 it can be seen that switch 200 connects both lamps 202 and 208 through the switches 96 and the switch 98, which is closed when the sensors 104 are inoperable, to power source 60 via resistor 220. Thus, before the individual leaves the building he can operate the momentary switches and if light 202 on the interior of the door is d) illuminated following depression of momentary switch 204 then he can be confident that the system is operable and ready to be activated. If light 202 does not, however, become illuminated following depression of normally open, momentary manual push button 204 then the system is not operable and further steps need to be taken to correct whatever problem has developed. Following operation of push button 204 and the viewing of the illuminated bulb 202 the individual then opens the door and steps outside. The tumbler 210 can then be operated by key 214 to rotate magnet 212 to a position shifting the position of the contacts in reed switch 213 and connecting together terminals A and C via lights 202 and 208, thus completing a current path through those lights between switch 98 and the transmitter circuit 64 so that a pure sinusoidal tone is generated and transmitted to telephone line 26 as described above. Terminal B is not used in a particular embodiment shown in FIG. 8 but could be employed in any instance in which a circuit was to be maintained while the premises is in an unprotected condition.

Many changes and modifications of the abovedescribed embodiment of the invention can of course be made without departing from the scope of the invention. Accordingly, that scope is intended to be limited only by the scope of the intended claim.

What is claimed is:

1. An alarm system comprising:

means at each of a plurality of alarm locations for providing an alarm signal upon detection of an emergency condition,

means connected to said alarm signal providing means for transmitting an alarm indicating signal,

central station means for receiving said alarm indicating signal and providing a display thereof,

line means connecting said transmitting means to said central station means, and

manually operable activating means connected to said transmitting means having a first condition permitting transmission of said alarm indicating signal and second condition preventing transmission of said alarm indicating signal including a housing, means for mounting said housing at the entrance and egress from a premise to be protected, a magnetically operable switch mounted within said housing, circuit means connecting said transmitting means to said line means via said switch so that said switch permits transmission in a first position and prevents transmission in a second position, key operated tumbler means and a magnet attached to said tumbler means adjacent said switch for causing said switch means to shift from said second to said first position when said key operated tumbler is operated so as to activate said transmitting means.

2. A system as in claim 1 wherein said activating means further includes means for mounting said housing on the interior surface of a door, means for mounting said tumbler means on the exterior surface of said door opposite said housing means.

3. A system as in claim 2 wherein said activating means further includes a first bulb mounted in said housing and visible from inside said door, a second bulb mounted in said tumbler mounting means and visible from the outside of said door, circuit means connecting said bulbs to said switch for illuminating said bulbs when said switch is in said second position, momentary switch means mounted on said housing and manually operable from within said door, and circuit means connecting said bulbs and said alarm signal providing means to said momentary switch for illuminating said bulbs when said momentary switch is manually operated and none of said alarm signal providing means is providing an alarm signal, thus indicating the system is functioning.

4. A system as in claim 1 wherein said transmitting means includes oscillator means for producing a single frequency sinusoidal output signal and means for coupling said output signal to said line means.

5. A system as in claim 4 including a plurality of said transmitting means each producing an output signal of a different frequency and each connected to said line means and wherein said central station means includes a plurality of filter means each connected to said line means and each responsive to one of the frequencies of said transmitters for producing a first signal when the signal on said line means includes a transmitted signal of that frequency and producing a second signal when the signal on said line means does not include a transmitted signal of that frequency, and a plurality of alarm means each connected to one of said filter means for providing an alarm indication whenever said second signal is produced by the connected filter means.

6. A system as in claim 5 wherein said filter means each includes means for maintaining said second signal after it is produced and a manually operable switch for permitting production by a filter means of a first signal after the signal on said line means again includes the frequency to which that filter means is responsive.

7. A system as in claim 6 wherein said maintaining means includes an SCR and wherein each said filter means further includes a source of power, means connected to said power source and said line means for receiving and filtering the signal on said line means and producing a D. C. level signal when the frequency to which that filter means is response is included, a transistor connected to said power source and said receiving, filtering and producing means so as to be conduc tive when said D. C. level signal is produced, means for connecting said SCR to said transistor so that said SCR is rendered conductive when said receiving, filtering and producing means no longer produces said D. C. level signal, and means connecting said switch and said alarm means in series with said SCR and said power source so that said alarm means remains operable until said switch is operated after said transistor has again become conductive.

8. A system as in claim 4 wherein said alarm signal producing means includes a plurality of alarm switches and said transmitting means further includes a source of electrical power, means connecting said oscillator circuit to said power source via said activating circuit and said alarm switches so that :said oscillator is rendered inoperative upon shifting of any one of said alarm switches to a position indicating an alarm condition.

9. A system as in claim 8 further including a capacitor connecting said alarm switches to said oscillator circuit.

110. A system as in claim 4 further including second means at each of a plurality of locations for providing an alarm signal upon detection of a second different alarm condition and means connected to said oscillator means and to said second means for alternatively rendering said oscillator means operative and inoperative to apply a modulated signal to said line means when said switch is in said first position and at least one of said second means is providing at alarm signal.

llll. A system as in claim 4 wherein said central station means includes means for detecting said modulating signal and providing an indication thereof.

12. A system as in claim 1 wherein said line means includes a telephone line.

13. A system as in claim 1 including a plurality of said transmitting means each transmitting a sinusoidal signal of a different frequency and each connected to said line means. 

1. An alarm system comprising: means at each of a plurality of alarm locations for providing an alarm signal upon detection of an emergency condition, means connected to said alarm signal providing means for transmitting an alarm indicating signal, central station means for receiving said alarm indicating signal and providing a display thereof, line means connecting said transmitting means to said central station means, and manually operable activating means connected to said transmitting means having a first condition permitting transmission of said alarm indicating signal and second condition preventing transmission of said alarm indicating signal including a housing, means for mounting said housing at the entrance and egress from a premise to be protected, a magnetically operable switch mounted within said housing, circuit means connecting said transmitting means to said line means via said switch so that said switch permits transmission in a first position and prevents transmission in a second position, key operated tumbler means and a magnet attached to said tumbler means adjacent said switch for causing said switch means to shift from said second to said first position when said key operated tumbler is operated so as to activate said transmitting means.
 2. A system as in claim 1 wherein said activating means further includes means for mounting said housing on the interior surface of a door, means for mounting said tumbler means on the exterior surface of said door opposite said housing means.
 3. A system as in claim 2 wherein said activating means further includes a first bulb mounted in said housing and visible from inside said door, a second bulb mounted in said tumbler mounting means and visible from the outside of said door, circuit means connecting said bulbs to said switch for illuminating said bulbs when said switch is in said second position, momentary switch means mounted on said housing and manually operable from within said door, and circuit means connecting said bulbs and said alarm signal providing means to said momentary switch for illuminating said bulbs when said momentary switch is manually operated and none of said alarm signal providing means is providing an alarm signal, thus indicating the system is functioning.
 4. A system as in claim 1 wherein said transmitting means includes oscillator means for producing a single frequency sinusoidal output signal and means for coupling said output signal to said line means.
 5. A system as in claim 4 including a plurality of said transmitting means each producing an output signal of a different frequency and each connected to said line means and wherein said central station means includes a plurality of filter means each connected to said line means and each responsive to one of the frequencies of said transmitters for producing a first signal when the signal on said line means includes a transmitted signal of that frequency and proDucing a second signal when the signal on said line means does not include a transmitted signal of that frequency, and a plurality of alarm means each connected to one of said filter means for providing an alarm indication whenever said second signal is produced by the connected filter means.
 6. A system as in claim 5 wherein said filter means each includes means for maintaining said second signal after it is produced and a manually operable switch for permitting production by a filter means of a first signal after the signal on said line means again includes the frequency to which that filter means is responsive.
 7. A system as in claim 6 wherein said maintaining means includes an SCR and wherein each said filter means further includes a source of power, means connected to said power source and said line means for receiving and filtering the signal on said line means and producing a D. C. level signal when the frequency to which that filter means is response is included, a transistor connected to said power source and said receiving, filtering and producing means so as to be conductive when said D. C. level signal is produced, means for connecting said SCR to said transistor so that said SCR is rendered conductive when said receiving, filtering and producing means no longer produces said D. C. level signal, and means connecting said switch and said alarm means in series with said SCR and said power source so that said alarm means remains operable until said switch is operated after said transistor has again become conductive.
 8. A system as in claim 4 wherein said alarm signal producing means includes a plurality of alarm switches and said transmitting means further includes a source of electrical power, means connecting said oscillator circuit to said power source via said activating circuit and said alarm switches so that said oscillator is rendered inoperative upon shifting of any one of said alarm switches to a position indicating an alarm condition.
 9. A system as in claim 8 further including a capacitor connecting said alarm switches to said oscillator circuit.
 10. A system as in claim 4 further including second means at each of a plurality of locations for providing an alarm signal upon detection of a second different alarm condition and means connected to said oscillator means and to said second means for alternatively rendering said oscillator means operative and inoperative to apply a modulated signal to said line means when said switch is in said first position and at least one of said second means is providing at alarm signal.
 11. A system as in claim 4 wherein said central station means includes means for detecting said modulating signal and providing an indication thereof.
 12. A system as in claim 1 wherein said line means includes a telephone line.
 13. A system as in claim 1 including a plurality of said transmitting means each transmitting a sinusoidal signal of a different frequency and each connected to said line means. 